This invention relates to an insulating resin composition and semiconductor device using the same, and more particularly it relates to a semiconductor device having a multi-level interconnection suitable for high integration.
Heretofore, in the production of a semiconductor device having a multilevel interconnection composed mainly of a two-level interconnection, a first level interconnection layer is at first formed on the base of the semiconductor, followed by forming an insulating film thereon, then forming a through-hole at a predetermined part of the insulating film according to a known hot etching process to expose a portion of the first level interconnection layer, then forming a metal film which is to constitute a second level interconnection layer according to vacuum deposition or spattering method and further subjecting this film to hot etching to form the second level interconnection layer. At that time, a metal film composed mainly of aluminum has most often been used as the interconnection layers, and an inorganic insulating film composed mainly of SiO.sub.2 film or an organic resin insulating film of polyimide group resins has been used as the insulating film material.
Since the interconnection layer is composed mainly of aluminum, it is necessary for preventing fusion of aluminum and running-through of fused aluminum into the joint of the semiconductor to restrict the temperature at which the insulating film is formed on the interconnection layer to 450.degree. C. or lower. Thus, the inorganic insulating film such as those of SiO.sub.2, silicon carbide, etc. should be formed at a relatively low temperature such as about 400.degree. C. according to chemical gas phase growth method, high frequency spattering method or the like. However, such methods have had the following drawbacks. According to the chemical gas phase growth method, SiO.sub.2 film formed is liable to cause cracks. So that a film having a thickness of at most only 1 .mu.m or less can be formed. Further according to the spattering method, the resulting inorganic insulating film has a low formation rate.
Further the inorganic insulating film have had the following drawbacks. Since the film is formed faithfully reproducing the projections and depressions (step differences) of the lower interconnection layer as it is, it has an inferior step-covering ability and the upper interconnection layer is liable to cause disconnection on the lateral side of the step so that it is deficient in reliability.
In order to overcome such drawbacks of the inorganic insulating film, a process has been carried out wherein an organic resin film of polyimide group resin including polyimide having fluidity and a superior step-covering ability is formed as an insulating film on a interconnection layer to planarize the projections and depressions on the lower interconnection layer and thereby dissolve the problem of the step difference. As materials for such polyimide (a cured product of a polyamic acid polymer obtained by reacting an aromatic diamine with an aromatic tetracarboxylic acid dianhydride) or polyimide group resin (e.g. a cured product of a polyamic acid polymer obtained by reacting an aromatic diamine, an aromatic tetracarboxylic acid dianhydride and an aromatic diaminocarbonic amide), for example, PIQ varnish (trademark of a product manufactured by Hitachi Chemical Company, Ltd.), etc. are used. This PIQ varnish is rotation-applied onto a lower interconnection layer, followed by volatizing the solvent component and then heating the resulting material to 200.degree. to 400.degree. C. to form a cured film of PIQ. This varnish have been usually adjusted to a resin content of 10 to 20% by weight and a viscosity of 5 to 50 poises.
As shown in FIG. 1, when the thickness of an interconnection layer 3 formed on a semiconductor base 1 is denoted as ta and the thickness of a step-covering part remained after formation of an organic resin insulating film 4 is denoted as tb, the value of ##EQU1## will be defined as a step-covering performance or a planarizing capability. The planarizing capability in the case where the polyimide or polyimide group resin is used is in the range of 0.15 to 0.4.
When an organic resin insulating film is formed and thereafter a through-hole is formed at a predetermined part of the resin insulating film in order to connect the lower interconnection layer and the upper interconnection layer, a wet etching process using a basic etching solution containing hydrazine is employed. Since this wet etching process is an isotropic etching which proceeds at the same rate in both the longitudinal direction and the lateral direction, there is practically no problem for the present with the polyimide or polyimide group resin having a planarizing capability of 0.15 to 0.4. However, there is still a difference in the film thickness between on the plane part and on the step-covering part, and further in the wet etching process, a proper etching time is different on the plane part and on the step-covering part, respectively. That is, the film thickness on the step-covering part is smaller than that of the plane part, so that when the etching on the plane part has reached a proper state, over-etching occurs on the step-covering part. Thus, the etching accuracy becomes inferior and hence when the polyimide or polyimide group resin is used as the insulating film material, a through-hole of 5 .mu.m square has been a practical limit.
On the other hand, accompanying a tendency of high integration of semiconductor devices, an interconnection of the device has been made finer year after year. Accordingly, for effecting patterning of the interconnection layer and opening the resin insulating film, a wet etching process has been changed to a dry etching process such as plasma or reaction type ion etching process on the like, which makes fine etching possible. For carrying out etching for a fine through-hole of 2 .mu.m or less according to these dry etching processes, it is necessary to expose a photoresist so as to give a high resolution, and it is preferred to planarize the step as much as possible which is caused by covering a lower interconnection layer with an organic resin insulating film. For this purpose, it is required to retain the thickness of the resulting film of conventional polyimide or polyimide group resins to the same extent, raise the content of the resin in order to enhance the step-covering capability and reduce the viscosity (or molecular weight).
However, conventional polyimide or polyimide group resins of polyamic acid polymer type have limit to elevation of the content of resin and reduction of the viscosity. Hence when conventional polyimide or polyimide group resins are formed on the interconnection layer, followed by etching the resin insulating film according to dry etching, the resulting etching accuracy is inferior; thus it has been difficult to obtain a semiconductor device having a more highly integrated multilevel interconnection structure.